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RESOLVER(5)		    BSD File Formats Manual		   RESOLVER(5)

     resolver — DNS client


     The resolver is the DNS client used on most Linux and BSD systems. It
     comes with glibc.	Its configuration file /etc/resolv.conf (note the
     spelling) determines the DNS servers to use, and various other options -
     see below.

     Almost all machines have a DNS server set up in this file - if it doesn't
     exist, the system will assume there's a DNS server running on the local
     machine, and work out the search path from the machines domain name.

     The config file is read the first time the DNS client is invoked by a

     The different configuration options are:

     nameserver	 IP address of a DNS server to use. Multiple name servers may
		 be listed, each on their own line. The resolver will use them
		 in order listed - if the first server times out answering the
		 query, the next server will be tried, and so on. If the
		 resolver runs out out of name servers, the first server will
		 be queried again, until a maximum number of retries are made.

		 The maximum number of DNS servers to use is set by MAXNS (see
		 <resolv.h> )

     search	 Domain(s) to use for DNS lookups when no domain is specified.
		 List each domain following the search keyword with spaces or
		 tabs between them. Each possible domain will be checked in
		 order until a match is found. Note that this process may be
		 slow (queries will time out if no server is available for a
		 domain) and will generate a lot of network traffic if the
		 servers for the listed domains aren't local.

		 The search list is currently limited to six domains with a
		 total of 256 characters.  If search isn't specified, the
		 search list will be determined from the local domain name
		 (whatever comes after the first dot). If the host name
		 doesn't contain a domain, the root domain is used.

		 By default, it search contains only the local domain name.

     domain	 Local domain name. You can use this instead of the search
		 option to specify a single domain to check if a hostname
		 isn't specified. Most people just use search instead (that
		 option lets you use multiple servers, domain doesn't). You
		 can't use domain and search at the same time - they're mutu‐
		 ally exclusive.

		 If domain isn't specified, the domain will be determined from
		 the local domain name (whatever comes after the first dot).
		 If the host name doesn't contain a domain, the root domain is

     sortlist	 Sorts addresses returned by the gethostbyname system call.  A
		 sortlist is specified by IP address netmask pairs. The net‐
		 mask is optional and defaults to the natural netmask of the
		 net. The IP address and optional network pairs are separated
		 by slashes. Up to 10 pairs may be specified.  For example:


     options	 Allows certain internal resolver variables to be modified.
		 The syntax is
		       options option ...
		 where option is one of the following:

		 debug	   sets RES_DEBUG in _res.options.

		 ndots:n   sets a threshold for the number of dots which must
			   appear in a name given to res_query() (see
			   resolver(3)) before an initial absolute query will
			   be made.  The default for n is “1”, meaning that if
			   there are any dots in a name, the name will be
			   tried first as an absolute name before any search
			   list elements are appended to it.

			   sets the amount of time the resolver will wait for
			   a response from a remote name server before retry‐
			   ing the query via a different name server.  Mea‐
			   sured in seconds, the default is RES_TIMEOUT (see
			   <resolv.h> ).

			   sets the number of times the resolver will send a
			   query to its name servers before giving up and
			   returning an error to the calling application.  The
			   default is RES_DFLRETRY (see <resolv.h> ).

		 rotate	   sets RES_ROTATE in _res.options, which causes round
			   robin selection of nameservers from among those
			   listed.  This has the effect of spreading the query
			   load among all listed servers, rather than having
			   all clients try the first listed server first every

			   sets RES_NOCHECKNAME in _res.options, which dis‐
			   ables the modern BIND checking of incoming host
			   names and mail names for invalid characters such as
			   underscore (_), non-ASCII, or control characters.

		 inet6	   sets RES_USE_INET6 in _res.options.	This has the
			   effect of trying a AAAA query before an A query
			   inside the gethostbyname function, and of mapping
			   IPv4 responses in IPv6 ``tunnelled form'' if no
			   AAAA records are found but an A record set exists.

		 ip6-dotint / no-ip6-dotint
			   sets / clears the RES_NOIP6DOTINT bit in
			   _res.options, which when set (ip6-dotint) will
			   enable reverse IPv6 lookups to be made in the (dep‐
			   recated) zone; when clear (no-ip6-dotint),
			   reverse IPv6 lookups are made in the zone
			   by default.

			   sets RES_USEBSTRING in _res.options.	 This causes
			   reverse IPv6 lookups to be made using the bit-label
			   format of RFC 2673; if not set, then nibble format
			   is used.

     The domain and search keywords are mutually exclusive.  If more than one
     instance of these keywords is present, the last instance wins.

     The search keyword of a system's resolv.conf file can be overridden on a
     per-process basis by setting the environment variable “LOCALDOMAIN” to a
     space-separated list of search domains.

     The options keyword of a system's resolv.conf file can be amended on a
     per-process basis by setting the environment variable “RES_OPTIONS to a
     space-separated list of” resolver options as explained above under

     The keyword and value must appear on a single line, and the keyword
     (e.g., nameserver) must start the line.  The value follows the keyword,
     separated by white space.

     /etc/resolv.conf <resolv.h>

     gethostbyname(3), hostname(7), named(8), resolver(3), resolver(5).	 “Name
     Server Operations Guide for BIND”

4th Berkeley Distribution	 June 23, 2004	     4th Berkeley Distribution

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